Alumina is chemically stable and has a high melting point and excellent physical properties such as electrical insulation, mechanical strength and hardness, and is thus widely used in various industrial fields such as refractories, abrasives and high-speed cutting tools. In particular, highly porous alumina is industrially and widely used in catalyst supports, adsorbents and various surface coating agents, and a range of required properties, such as a pore volume, a pore size, a pore size distribution, and a specific surface area, vary depending on the purpose of use thereof. Recently, due to trends of diversification of industries, a demand for highly porous alumina having a large pore volume is increasing. Crystalline alumina as gibbsite, bayerite and boehmite may be prepared according to the method of preparing alumina, or amorphous alumina may be prepared. Among them, a boehmite or pseudoboehmite structure is known as the most preferred structure to control the pores thereof. Accordingly, studies on the method of preparing a porous alumina having a boehmite or pseudoboehmite structure have been performed from many different perspectives.
Currently known methods of preparing a porous alumina having a boehmite or pseudoboehmite structure are as follows.
U.S. Pat. No. 7,442,361 discloses that alumina having a boehmite or pseudoboehmite structure is prepared by performing a method of using crystal seeds having an average diameter from 20 nm to 150 nm and a precipitant such as an alkali metal or aluminum sulfate, and adjusting the pH from 6 to 8. In U.S. Pat. No. 7,442,361, different crystal seeds are applied depending on the properties required for alumina, and thus the process needs to undergo a change. That is, the limitation of raw materials and the resultant diversification of process facilities follow.
Further, Japanese Patent Application Publication No. 2003-313027 discloses a method of preparing a boehmite sol, the method including: obtaining an alumina hydrate slurry in a batch-type manner by slowly adding a sodium aluminate solution to an aluminum chloride solution at room temperature until the pH becomes 10 or more, preparing a boehmite slurry by hydrothermally synthesizing the alumina hydrate slurry, filtering and washing the boehmite slurry, and then peptizing the alumina hydrate slurry by using a nitric acid aqueous solution such that the pH becomes 4. In Japanese Patent Application Publication No. 2003-313027, the uniformity of a product deteriorates because a batch-type processing process is performed while the pH is adjusted to 10 or more, and there is a problem in process facilities and process safety because a strong acid is used in the peptization process.
In addition, U.S. Pat. No. 6,048,470 discloses a method for producing an alumina sol, the method including: stirring a dispersion of an alumina hydrate having a solid content of from 1 wt % to 40 wt % at a pH of from 7 to 12 with an effective consumptive power of 0.5 kW/m or more for aggregation treatment, and then adding an acid thereto for peptization. U.S. Pat. No. 6,048,470 includes the contents of controlling the pH of a dispersion of an alumina hydrate to a predetermined range by using an alkali metal hydroxide, and includes a process of obtaining alumina having small particles by repeating eight times a step of adjusting the pH to 3.5 by using an aluminum nitrate solution having a concentration of 5 wt % and adjusting the pH to 9.5 again by using a sodium aluminate solution. Therefore, U.S. Pat. No. 6,048,470 has the complexity of a process in which the pH adjustment process is repeated, and thus has low economic efficiency for commercialization.
Furthermore, European Patent No. 0934905 discloses a method of preparing alumina having a boehmite structure, the method including: a process of dispersing an alumina hydrate in an acidic solution having a pH from about 3 to about 4, and then controlling the pH from 10 to 11 by using an alkali reagent, a process of obtaining a colloidal sol by aging the dispersion at a temperature of 80° C. or more for at least 4 hours or more, and adjusting the pH to 8 by using an acidic reagent, and a process of peptizing the colloidal sol. In European Patent No. 0934905, an aging process is performed as a means for controlling the pore size, volume and surface area of alumina, and the process time becomes long as it takes 35 hours for the aging, and therefore, there is a problem of a reduction in productivity and an increase in process costs. As an invention which discloses a similar aging method, Japanese Patent Application Publication No. Sho 53-19000 discloses a method of growing an amorphous alumina hydrate to an alumina having a pseudoboehmite structure with a pore volume more than 0.6 mL/g and an average pore size of 600 Å or less by aging the amorphous alumina hydrate at a pH from 8 to 12 and a temperature of 50° C. or more. Japanese Patent Application Publication No. Sho 53-19000 has a problem in that productivity is reduced by an aging process at a low temperature for a long time.
Further, Japanese Patent Application Publication No. Sho 58-190823 discloses a method of forming an alumina gel by adding a pH adjusting agent and an aluminum salt solution to an aluminum hydroxide-containing slurry at a predetermined rate while maintaining a pH from 6 to 11 and a temperature of 50° C. or more, and converting the prepared alumina gel into an alumina having a boehmite structure. Japanese Patent Application Publication No. Sho 58-190823 has a problem in that it is difficult to stably produce an alumina having a uniform particle size distribution because a precipitate of alumina hydrate is produced in the aluminum hydroxide-containing slurry so that that the mixture state of the entire reactant is not uniform, and the composition of the prepared alumina product is also non-uniform.
As described above, studies on a method for preparing an alumina having a boehmite or pseudoboehmite structure have been performed from many different perspectives, but still have limitations for commercial use.